A brushless DC motor has a rotor having a permanent magnet with a plurality of poles and a stator having coils of three phases of a plurality of coil phases, for example, U, V, and W. Upon conduction of electricity to a coil with a phase corresponding to the position of the rotor with respect to the stator, the rotor is rotated. A rectangular wave driving technique is relatively a simple technique for the driving control. It is a driving technique for sequentially switching coil phases to be conducted with electricity at every rotation of the rotor by 120 degrees. According to this technique, the permanent magnet of the rotor receives a crossing magnetic field by the coil of the stator, and generation of the rotation magnetic field is controlled for rotation in a predetermined direction at a predetermined speed. According to this technique, for switching the coil phases to be electrically conductive one by one, it is necessary to detect the rotation position of the rotor with respect to the stator one by one.
Japanese Unexamined Patent Application Publication No. 2003-319686 discloses a technique for absolutely detecting the rotation position based on a counter electromotive force of the motor. A motor current is compared with a set current value, using a current comparator unit for detecting the counter electromotive force in two stages, that is, at the elapsed point of a set time of a first ON section set timer and from and after the elapse of a set time of a second ON section set timer. That is, based on a comparison result at the elapsed point of the set time of the first ON section set timer, it is immediately switched to an OFF section, and the minimum ON section is shortened, and the counter electromotive force is not detected. On the other hand, the ON section is kept after the comparison, and the counter electromotive force is absolutely detected, in a section after the elapse of the set time of the counter electromotive force detection permission timer until the elapse of the set time of the second ON section set timer. As a result, when a current value has already reached a comparison reference signal in a counter electromotive force non-detection period, it is possible to avoid a case in which the detection of the counter electromotive force is not possibly performed, thus contributing to avoid an undesired possibility that the rotation position cannot be detected.